Current trends in which patterns for integrated circuits and displays are being made increasingly larger have resulted in the reticle pattern frequently being larger than the field of the exposure-optical system of the microlithography apparatus. This situation especially arises in charged-particle-beam (CPB) microlithography apparatus in which the field of the exposure-optical system typically is very small compared to the area of the reticle. In situations in which the pattern is larger than the optical field, the pattern is usually divided into multiple individual exposure units (e.g., "subfields") that are separately exposed in an ordered manner to transfer the entire pattern. Microlithography performed using such a reticle is termed "divided" transfer-exposure.
During transfer of individual exposure units in divided transferexposure, the reticle and substrate (which are mounted on respective movable stages) are moved in a coordinated manner as required in respective planes that are perpendicular to the optical axis of the exposure-optical system. Also, as each exposure unit is exposed, one or more of certain parameters of the exposure-optical system (e.g., focus, image magnification, aberration correction, etc.) are optimized for the respective exposure field. The coordinated movements of the reticle stage and substrate stage can be according to either a "step-and-repeat" or a "continuous scanning" scheme. In step-and-repeat exposure, the stages move intermittently (e.g., to position the next exposure unit for exposure) and no exposures are made while the stages are moving. Rather, an exposure is made (of the positioned exposure unit) only when the stages are stationary. In continuous scanning exposure, exposures are made while the stages are moving at respective scanning velocities.
In CPB microlithography, the reticle can be a so-called "stencil" reticle or a so-called "membrane" reticle. With a stencil reticle, pattern features (i.e., pattern elements) are defined as respective through-apertures in a reticle plate. Certain features, such as "island" features, cannot be completely defined using a single respective aperture in a stencil reticle. Such features are usually divided into two complementary features defined in separate exposure units that are transferred onto the substrate in separate respective exposures ("shots").
As is generally known, a current quest in integrated circuit technology is the manufacture of ever-larger memory chips. According to contemporary "roadmaps" of memory circuits, the anticipated dimensions of a 16-gigabit DRAM chip are approximately 40.times.20 mm. As explained in more detail below, the dimensions of a reticle (assuming a demagnification factor of 1/4 and no complementary exposure units required) for such a chip would exceed 200.times.100 mm. If the pattern required any complementary exposure units, then the reticle would be even larger, and likely would not fit on a 12-inch diameter reticle plate.